ABA Billing CO-24 Duplicate Denial: How to Appeal and Win

The CO-24 denial reason code (“charges are covered under a capitation agreement/managed care plan” or “duplicate of a previously adjudicated claim”) is one of the most misunderstood and most frequently misapplied denials we see hitting ABA practices in 2026. Across roughly 50 behavioral health practices in our current book of work, CO-24 accounts for between 8% and 14% of all first-pass denials on ABA claims, and the average practice is recovering fewer than 40% of those dollars without a structured appeal process in place.

The frustrating reality is that most CO-24 denials on ABA claims are payer error, system error, or a configuration problem that your clearinghouse is either masking or amplifying. They are rarely a true duplicate. This post walks through exactly why CO-24 fires on ABA claims, what documentation you need to overturn it, and the specific workflow changes that push recovery rates above 70% at practices billing 200 or more ABA sessions per week.

Why CO-24 Fires on ABA Claims More Than Any Other Specialty

ABA is uniquely vulnerable to CO-24 because of how the services are structured. A child receiving intensive ABA may have four or five CPT codes billed on the same date of service: 97153 (adaptive behavior treatment by protocol), 97155 (protocol modification by a BCBA), 97156 (family adaptive behavior treatment guidance), and 97158 (group adaptive behavior treatment) can all legitimately appear on the same claim or on split claims from the same provider group. Payer claim adjudication systems that were configured before the ABA CPT code set was standardized in 2019 still flag same-date, same-patient claims from the same NPI as duplicates when they are not.

We also see CO-24 fire in these specific scenarios:

• Split billing between BCBA (97155) and RBT (97153): When the supervising BCBA’s service and the technician’s direct service are billed on the same date from the same billing NPI, some payers collapse them into a single claim review and deny one as a duplicate.
• POS code inconsistency: A claim billed with POS 12 (home) and a resubmission corrected to POS 11 (office) or POS 02 (telehealth) will trigger CO-24 if the payer links them by date and member ID rather than by claim ID.
• Clearinghouse rebatch on rejection: If a claim rejects at the clearinghouse level and is resubmitted without a new original claim indicator, the second submission reads as a duplicate at adjudication.
• Coordination of benefits between Medicaid and a commercial plan: When ABA is covered under both a primary commercial plan and Medicaid waiver, the secondary payer frequently fires CO-24 rather than applying COB logic correctly.

The Dollar Impact of Unworked CO-24 Denials at Different Practice Sizes

We have pulled denial data across small, mid-size, and large ABA practices and the revenue exposure from unworked CO-24 accounts is significant and scales predictably with session volume.

• Small practice (under 50 sessions per week, 1-3 BCBAs): Average CO-24 exposure runs $2,800 to $5,500 per month. At this volume, most practices are writing off the balance after one failed appeal attempt.
• Mid-size practice (100-200 sessions per week, 4-10 BCBAs): Monthly CO-24 exposure typically lands between $12,000 and $28,000. This is where the write-off habit compounds into a six-figure annual loss.
• Large multi-site ABA group (200+ sessions per week): We see CO-24 denial buckets reaching $40,000 to $90,000 per month across some payer panels, particularly with Medicaid managed care organizations that use aggressive duplicate claim logic.

The practices recovering 70% or more of CO-24-denied revenue share two characteristics: they appeal within 30 days of the denial date, and they send a different document set on the appeal than they sent on the original claim.

Exact Documentation Needed to Win a CO-24 ABA Appeal

Sending the original claim again is the most common and most damaging appeal mistake we see. The payer already adjudicated that claim. Your appeal needs to prove that what looks like a duplicate is a clinically and procedurally distinct service. Here is the document set that we use in appeals that achieve above-average overturn rates:

• Session notes with timestamps: For 97153 and 97155 billed on the same date, you need session documentation showing that the direct treatment time (97153) and the protocol modification time (97155) occurred in non-overlapping time segments. Timestamps matter. Narrative notes without clock times will not satisfy most payer reviewers.
• The authorization record: Pull the authorization showing both CPT codes were authorized for the same date of service. This is your first line of evidence that the services were expected and planned, not duplicated.
• The original EOB showing the first adjudicated claim: If the payer has already paid one of the codes and is calling the second one a duplicate, attach the EOB proving the two codes are distinct line items, not the same service submitted twice.
• A written letter of medical necessity: For appeals past the first level, the BCBA’s clinical rationale for billing multiple codes on the same date strengthens the case substantially. Most payers require this at the second level of appeal.
• The claim activity report from your clearinghouse: This proves the claim was submitted once with the correct claim frequency indicator (typically “1” for original). If a rebatch happened, it documents the reason and timing so you can argue against duplicate submission classification.

Modifier and Billing Configuration Fixes That Stop CO-24 Before It Starts

The most efficient recovery strategy is prevention. After working appeals retroactively, these are the system-level fixes that reduce CO-24 fire rates at the practices we manage:

• Modifier HN and HO: When billing 97153 (RBT-delivered) and 97155 (BCBA-delivered) on the same date, append modifier HN (bachelor’s level) or HO (master’s level) to differentiate the rendering provider credential at the line level. Some payers route these to different claim review logic when the modifier is present.
• Separate claims by rendering NPI: Where payer policy allows, bill the RBT’s 97153 units under the RBT’s individual NPI (or supervised provider NPI) and the BCBA’s 97155 under the BCBA’s NPI. This forces the payer’s system to evaluate two claims from two providers rather than collapsing them.
• Claim frequency indicator discipline: Train your billing team to use frequency indicator “7” (replacement of a prior claim) on corrected resubmissions and to always include the original claim number in field 22 of the CMS-1500. This is the most common preventable cause of CO-24 in ABA billing.
• Payer-specific duplicate claim windows: Some Medicaid MCOs define a “duplicate” as any claim with the same CPT, same member, same DOS, and same rendering NPI within a 30-day lookback window regardless of units. Know your top five payers’ duplicate logic before you bill.

Appeal Timeline and Escalation Path When First-Level Is Denied

CO-24 appeals that are not worked within the payer’s timely filing window for appeals (which is separate from and usually shorter than the timely filing window for claims) are almost never overturnable. The standard window is 90 to 180 days from the denial date depending on payer, but several Medicaid MCOs in 2026 are enforcing 60-day appeal windows. Set a hard 30-day internal deadline to submit your first-level appeal so you have room for escalation.

If first-level appeal is denied again, your escalation path should be: second-level internal appeal with clinical documentation, then external independent review if the payer is subject to state prompt pay and appeals regulations. For CO-24 denials rooted in payer system error (where the payer’s own EOB shows no prior payment for the denied code), many states allow providers to file a complaint with the state insurance commissioner, which often accelerates resolution faster than the formal appeal track. This intersects with broader payer accountability issues we have covered in our analysis of mental health parity act appeals and how behavioral health practices are leaving money on the table, particularly when CO-24 is being used by payers to circumvent ABA coverage mandates.

What to Track in Your Denial Dashboard for CO-24 Specifically

If your practice management system or RCM platform is not tracking CO-24 denials as a distinct denial reason code category with payer-level, CPT-level, and DOS-level breakdowns, you are managing this problem blind. The metrics that matter for CO-24 specifically are: denial rate by CPT code (you will usually find 97155 and 97153 are the top two CO-24 triggers), first-pass denial rate by payer, appeal submission rate within 30 days, overturn rate by appeal level, and average days to resolution. Practices that build this dashboard recover 1.8x more CO-24 revenue in the first 90 days compared to those tracking only aggregate denial rates.

This same discipline around denial segmentation applies across all denial types in behavioral health. If your SUD practice is also experiencing coding-level denials, our post on G0480-G0483 drug screen coding and why most SUD practices are under-coding covers a similar pattern of recoverable revenue that most practices do not realize they are losing.

Take Action on Your CO-24 Denials This Week

CO-24 is one of the highest-recovery denial categories in ABA billing when it is worked correctly and worked fast. The practices losing the most to it are the ones treating it as a write-off category rather than a billable appeals opportunity. If you want to see exactly how much CO-24 revenue your practice is leaving on the table, we offer a free 30-day denial audit that segments your denial volume by reason code, payer, and CPT, and shows you a prioritized recovery plan. Schedule your audit directly at our billing audit calendar and let us show you what your CO-24 bucket actually looks like.